1. Field of the Invention
The present invention relates to an output control system for automobile engines, which regulates the air intake into the engine by opening and closing the throttle valve of an automobile engine under electronic control and thereby controls the power output of the engine.
2. Description of Related Art
In a conventional vehicle, as the driver steps on the accelerator, the cable connected to the accelerator is operated according to the depth to which the pedal is pressed down, the throttle valve of the air suction pipe of the engine is thereby mechanically operated, and the air intake of the engine is thereby regulated to control the power output of the engine.
In recent years, along with the advancement of electronic engine control technology, interest has come to be focused on electronic throttle control systems which electrically detect the depth to which the driver presses down the accelerator, and electrically operate the throttle valve according to-the detected depth to control the output of the engine.
Examples of the prior art pertaining to such electronic control systems include the "engine output control method" disclosed in the Japanese Patent Laid-open No. 201061/1990, whereby the target torque of the engine is calculated from the degree of accelerator opening, vehicle speed and engine revolutions, and the throttle valve is operated by an electric motor in accordance with the result of calculation to control the output of the engine.
1. Fine Control of the Air Intake
In order to satisfy the official restrictions on various emissions, which are becoming increasingly stringent every year, by such a way of engine output control, the engine should be subjected to highly sophisticated combustion control, and this in turn requires fine control of the air intake which determines the condition of combustion.
An engine control apparatus using this method is enabled to accurately control the air/fuel ratio (the ratio between the quantity of air sucked into the engine and that of fuel), which impacts the characteristics of the exhaust gas from the engine by regulating not only the quantity of fuel fed to the engine but also the air intake to the engine with an electronic throttle control system, and it has been made possible to reduce gas emissions by achieving an ideal condition of combustion in each state of vehicle driving.
2. Engine Torque Control With a High Degree of Freedom
While the use of such an electronic throttle control system has made it possible to control the engine output matched with the driver's driving action according to the running state of the vehicle, it is also made possible to set the output control as desired in compliance with the requirement of the engine and thereby to achieve desired engine output characteristics with a high degree of freedom.
Furthermore, in recently developed engines wherein combustion takes place with a very dilute fuel-air mixture, such as in-cylinder fuel-injected engines, seemingly in contradiction with the requirements for reduced fuel combustion, suppressed emissions and enhanced power output, when switching is done between normal combustion and combustion with a dilute fuel-air mixture with the degree of accelerator opening kept constant, i.e. with the driver's demand for engine torque kept constant, it is necessary to control the air intake to the engine in order to restrain fluctuations in engine output torque. Compared with earlier dilute combustion engines, in-cylinder fuel-injected engines entail a broader range of air intake control because the fuel-air mixture is much more dilute, and the air intake accordingly requires even finer control than before.
3. Collective Control of Engine Output
For controlling the air intake (torque) of the engine, the prior art provides an apparatus to control the idling of the engine by regulating the quantity of air feed through an air passage which bypasses the throttle valve during idling i.e. when the throttle valve is closed.
Further, for controlling the output of the engine by regulating the throttle valve only in a specific state of driving, there are available a cruise control apparatus for regulating the vehicle speed at a driver-set level by feedback, and a traction control apparatus for restraining the engine output if the tires slip when starting on a slippery road.
These ways of engine output control make possible collective control with an electric throttle valve (ETV) control device, and contribute to simplification of the overall system configuration of the engine and to highly precise engine output control.
Because of its extremely important function to control the output of the engine as stated above, the ETV control system is so designed as not to allow engine revolutions to rise abnormally even if the system fails.
More specifically, when the throttle valve controlling motor is made unable to operate normally by disconnection or short-circuiting in the driving circuit for the throttle valve controlling motor, disconnection of the motor coil or any other similar cause, the throttle valve is released from motor control by cutting off power supply to the throttle valve controlling motor, and limp-home operation is done with a mechanism to fix the throttle valve in a preset position mechanically. As a result, drivability during limp-home operation and safety during system failure can be enhanced.
The operation of an apparatus according to the prior art will be described below with reference to a timing chart presented herein as FIG. 2.
The timing chart of FIG. 2 illustrates, from a point of time T1 to T6, how the engine and the vehicle behave when the accelerator is opened from a totally closed state to APS 1 at the point of time T1, closed from APS 1 to the totally closed state at the point of time T3, and opened again from the totally closed state to the position of APS1 from the point of time T5 to T6.
First the number of revolutions of the engine until the point of time T1 indicates idling with the accelerator totally closed, and at the point of time T1, the shift lever is shifted from its neutral position to the drive range (D range) at the point of time T1, and along with that the throttle valve is opened by an ETV unit to TPS1. However, if, after a further shift to the second speed is effected by a transmission control unit (TCU) at the point of time T2, an abnormality occurs in the ETV system at a point of time TF and the throttle valve further opens to TPS 2, the engine revolutions (Ne) and the vehicle speed (Vs) will turn toward a higher speed range against the driver's will.
Although the driver becomes aware of the abnormality and moves his or her foot off the accelerator at the point of time T3, the throttle opening will be kept at TPS2 until the point of time T4 when an ECU detects the abnormality in the ETV system and turns off power supply to the motor driving the throttle valve. After that, the release of the throttle valve from motor control and its mechanical closing to a preset fail-safe opening (TPS3) bring down the vehicle speed (Vs) gradually from VS1, and the engine revolutions (Ne) also converge to Ne1.
After that, when the driver again steps on the accelerator at the point of time T5, as the TCU determines the step of speed change to be selected according to the accelerator opening and the engine revolutions, shifting will take place from the second to the first speed. However, a speed change takes place in a condition wherein the throttle valve opening remains stuck at TPS3 and moreover the air intake volume also is constant, and the vehicle speed once drops momentarily to VS2 with the shock of speed change and rises again, with the engine revolutions also rising to Ne2. After that, if the driver moves his or her foot off the accelerator, shifting will again take place to the second speed, and the engine revolutions will drop to Ne1.